oh, you asked about making one! hmmmm.......try using an empty half-gallon of milk, alot of cotton, and an un-lubricated condom. cut the end off the empty milk jug, load it with the cotton, and insert the condom to make a space for ur dick. use whatever to attach everything (glue?)

I also found this how-to that people were discussing on a forum. They say it's pretty good.

Instructions (I didn't write these):

I'll teach you how to make the BEST REALISTIC HOMEMADE FAKE VAGINA SIMULATOR in 5 easy steps! You'll be screaming with pleasure in like 10 min and I bet you can not last more than 5 minutes! Here is the aspirin for your combat head , just read on...

For this experiment we are going to need following "vagina parts" and say - sexual accessories:

1. one empty plastic bottle. Make sure the bottle is in good shape, reasonably firm and has bottle cap on.
2. one plastic bag - try 'n keep from garbage bags especially used ones. Blow and pump it first - no holes allowed.
3. wide sealing tape or even better 2 or 3 strong but elastic rubber tags
4. hot water (as hot as you want your homemade vagina to be hot!)
5. Some kind of lubricant (try something mild like baby oil and stay away from anything peppermint or alcohol based. Fake vagina is for fun, remember? )

Building your new toy: Mrs. Vagina

Step 1: Take the bottle, cut off the bottom, unscrew the cap.
Step 2: Take plastic bag and pull the bag bottom through the bottle neck so that 2 to 3 inches of the bag are over the top.
Step 3: Spread out and wrap the bag's open part over the lower part of the bottle surface and fix it to the bottle with whatever you have from vagina parts No. 3.
In just seconds your homemade vagina is ready to ride, but here is the hot part:
Step 4: Remember the bag bottom? Hold it with one hand and sip the hot water into the bottle! If you did good job in step 3, the water will stay between the bottle and the bag inside giving your artificial vagina simulator softness and it will imitate the real vagina heat! And remember to screw bottle cap back OVER those 2-3 inches of bag bottom!!!
Step 5: I guess you know what to do with lubricant...